The raid of a cash depot in Kent is another example of robbers kidnapping staff and their families in order to negotiate hi-tech security systems. So what can be done about it?

As technology makes security systems all the more impenetrable, one weak link in the chain of protection remains - the human being.

In a tactic known as "tiger kidnapping", due to the way the beast follows its prey before it strikes, the family of a senior member of staff is held hostage to force the employee to participate in a robbery.

Coerced, often at gunpoint, the worker takes the robbers through the coded alarm systems, in the knowledge that one false move could mean the end for his or her loved ones.

But however real the threat of violence and however terrifying the ordeal, the family are usually unharmed.

Tiger kidnapping requires a detailed knowledge of staff - their journeys, their responsibilities and their families - which often comes with help from a current or former employee.

Staff are the only vulnerability they have, with all the alarm systems and security measures

John O'ConnorFormer Scotland Yard boss

Although it's a method commonly associated with paramilitary groups such as the IRA breaking into banks in Northern Ireland, a heist on the British mainland sent shockwaves through the security industry as long ago as 1972.

The manager of the NatWest bank in Sunbury, Middlesex, was kidnapped and held hostage with his family overnight in Surbiton, recalls John O'Connor, former commander of the Scotland Yard's Flying Squad.

He was brought to the bank in the morning, when staff arriving at work were bundled aside while the vaults were cleared.

There have been plenty of other examples in the intervening decades, concerning banks and building societies, culminating in the Northern Bank raid in Belfast in 2004, which netted the perpetrators £26m.

"When the NatWest bank was robbed the security industry was in a bit of a flap about it because they all felt vulnerable to that and so set about developing methods to deal with it," said Mr O'Connor.

"It's very surprising that with all the experience of this happening, you would have thought Securitas would have put contingency plans in place to deal with it.

"Staff are the only vulnerability they have, with all the alarm systems and security measures. No-one's likely to get through just using sawn-off shotguns."

Distress signals

The long period of contact with the victims means the risk of detection is quite high, but the criminals would disguise themselves and may even use Irish accents to point police towards paramilitaries, said Mr O' Connor.

And the more crime scenes there are, including a family home in a residential street with neighbours close by, the more chance of leaving a trail.

A Securitas spokesman said: "Managing people in a security environment is always very difficult. We continually review our security to try and eliminate those weaknesses.

"Without doubt, in this instance, the people have been the weak link, but until we speak to those people we don't know if anything went wrong."

So what are the ways to counter tiger kidnaps?

One obvious way is to use flexible security systems which do not allow one person to have access to all the codes.

Other counter-kidnapping measures were outlined by one BBC News website reader.

He said: "My mother works in one of these cash depots and they are trained what to do in such an event, including what to do if they think they are being followed or are hijacked on the way into work, including using coded distress words or codes to access different areas of the building that are then triggered to police."

There are also identity cards being developed which contain the latest mobile phone technology and enable an employee to discreetly raise the alarm in a dangerous situation.

As well as having a tracking device, the cards can also act as a bug so a firm's security department can listen in on the conversations and alert the police.